The present invention relates to a method and arrangement for automatically measuring a workpiece on a coordinate measuring machine, in which the work-contact probe or scanning pin of the measuring machine is moved continuously along the surface of the workpiece, and in which electrical signal outputs of measurement transducers in the scanning head (upon deflection of the probe pin) are used both to form the measurement value and to control the machine drives.
Measurement methods which continuously scan the workpiece are generally referred to as scan or scanning methods. In known scanning methods, the scanning pin of the scanning head is clamped with respect to two coordinate directions and is moved in the direction of one of the clamped axes, while in the third coordinate the scanning pin is movable substantially perpendicular to the surface of the workpiece and slides along it. In order to maintain contact between workpiece and scanning pin and to maintain a constant force of workpiece engagement, the measurement transducer serving for the measurement of scanner deflection in this third coordinate is switched into the position-control circuit for the machine axis in question.
Since in this method the direction of scanning is always bound to a machine axis, it is necessary, if the angle between the direction of travel and surface of the workpiece is large and exceeds about 60.degree., to effect a "reclamping", i.e., the scanning direction and the measurement direction are interchanged; as a consequence, therefore, the measurement process must first of all be interrupted. Furthermore, the measurement force changes in the course of the scanning, due to substantial friction between the scanning pin and the workpiece, the friction being a function of the instantaneous angle between the scanning direction and the surface of the workpiece.
From West German published patent application OS No. 2,921,166, it is shown to effect an automatic switching between scanning direction and measuring direction when the control voltages in the control circuits of the drive axes for the measurement direction and for the scanning direction are the same.
In this known method, since scanning direction is also bound to a given drive coordinate of the machine, changes in the measurement force exerted on the workpiece also take place, thus affecting the result of the measurement, since only the components of the measurement force in the axis controlled by the scanning head are maintained constant; i.e., the resultant total measurement force is not held constant. The scanning process furthermore does not take place uniformly, since only the velocity in one drive direction is held constant; i.e., velocity varies along the actual path, and this variation is dependent on the geometry of the workpiece.